This invention relates in general to the treatment of fluid waste mixtures and, in particular, to apparatus for the separation and removal of oil and particulate solids simultaneously from a dispersion of oil and particulate solids in water.
The Environmental Protection Agency has set specific limits for the concentration of oil in water which can be safely placed into sewer systems without causing harmful effects to publicly owned sewage treatment plants. This concentration is usually 100 mg/l, although when there exists more stringent local restrictions, the more stringent limits take precedence. Local restrictions in harbor areas usually forbid the dumping of oily water in concentrations capable of producing an oil sheen. This oil concentration is approximately 10 mg/l. Therefore, in order to dispose of an oil and water mixture into a sewer system or dump into a harbor, the oil concentration must be reduced to either 100 mg/l or 10 mg/l, respectively.
Existing equipment used for in line removal of oil and suspended particles (dirt) from water typically consists of numerous filter elements and coalescer elements placed in series. The construction of both the filters and the coalescers resembles conventional cloth wound or fabric depth filters, whose dense, fibrous, tortuous path causes separation of the suspended particles in the filter elements, and likewise causes separation of the oil in the coalescer elements.
As an example, the existing method for in line removal of suspended particles (dirt) and oil from the bilge water discharge of a Navy vessel is to first filter the water through a bank of prefilter elements (approximately 10), then coalesce a percentage of the oil in the flow stream through a bank of coalescer elements (approximately 5), and finally coalesce the remaining oil through a second bank of coalescer elements (approximately 5). This flow scheme (prefilter.fwdarw.coalescer.fwdarw.coalescer) is necessary because presently used filters are not designed to remove oil and consequently become plugged prematurely by entrained oil particles. Likewise, coalescers are not designed to remove dirt and become plugged prematurely by entrained dirt particles. Once plugged, neither the filter elements nor the coalescer elements can be cleaned due to their thick, dense construction.
As a result, the filters and coalescers used have an extremely short operating life and must be changed frequently. Typically, filter elements operate for less than one hour before excessive pressure drop forces replacement, while the coalescer elements run somewhat longer before they too must be replaced. For example, while removing the contents of a 25,000 gallon capacity donut, the prefilter bank (10 filters) is changed four times while each of the coalescer banks (five coalescers per bank) is changed twice. At a material cost of $37 per prefilter and per coalescer, the cost of element replacement is $2,220 per 25,000 gallons or about $89 per 1000 gallons of oily water. This cost is greater than the cost of processes which can produce distilled water from a seawater source, long considered a very expensive method of producing water.